Ladles and tundishes used for teeming molten steel require an outlet or outlets at the bottom thereof to direct the flow of the molten metal into a subsequent stage, e.g. a tundish, inner mold, or continuous casting molds. These outlets are typically formed by special nozzles made of refractory material having good corrosion resistance. Control of the casting rates of the molten metal is generally carried out by means of either a stopper rod assembly or a slide gate system, both of which include similar refractory material. Conventional nozzles are typically alumina-silica, chrome-alumina, alumina-graphite or zirconia-graphite refractories. A problem with such materials is that they have an affinity for impurities in steel, especially in aluminum killed steels. In this respect, deposits are apt to chemically and/or mechanically attach to the inner bore surface of the nozzles and form deposits thereon. These deposits build-up to a point where they restrict flow, and sometimes block the orifice to such a degree that flow stops.
In an attempt to solve the blockage problems created by deposit build-ups, it has been known to use porous, gas permeable nozzles to introduce an inert gas into the bore. Permeable nozzles known heretofore generally include a refractory and a metal jacket or housing spaced therefrom, wherein an air space or manifold is defined therebetween. Gas is introduced into the space or manifold through a fitting in the metal jacket. Pressure builds up between the refractory and the jacket, until it reaches a pressure sufficient to overcome the resistance inherent in the permeable refractory, at which point the inert gas flows through the refractory into the nozzle bore. Ideally, the introduction of the inert gas creates a gas film along the inner surface of the bore to retard deposit build-up. (An additional advantage of using inert gas is that it creates a positive pressure which prevents introduction of air into the molten metal. This prevents oxidation of the metal.) However, these devices are not capable of directing greater gas flow to specific locations in the bore where the build-up of deposits is most prevelant. Moreover, while maintaining an inert gas film on the bore of the nozzle increases nozzle life by retarding the build-up of deposits thereon, it does not completely eliminate the chemical and/or mechanical attraction between conventional nozzle refractory material and the impurities in the molten steel. In this respect, most conventional nozzles are alumina-silica based and have a strong affinity for impurities found in steel. Other materials, such as magnesium oxide (MgO), which is known to have no affinity for alumina, has found little acceptance or use in the manufacture of nozzles. With respect to magnesium oxide (MgO), its disfavor may be due to a perceived tendency to cracking.
In any event, the chemical attraction between impurities in molten steel and material found in conventional nozzles, together with the physical shape of the nozzle orifice (which may include areas or shapes which facilitate deposit build-up) tend to limit nozzle life.
The present invention overcomes these and other problems and provides a nozzle for teeming molten steel having a substantially reduced affinity for alumina and other impurities within the molten metal, which nozzle is porous and has a high degree of gas permeability and which provides greater gas flow to specific areas within the nozzle.